Health and Quality of Life Outcomes 2012, 10:118
doi:10.1186/1477-7525-10-118

Published: 25 September 2012

Abstract

Background

Index measures for health-related quality of life (HRQoL) quantify the desirability
(utility) of a certain health state. The commonly used generic index measure, e.g.
EuroQol: EQ-5D, may underestimate relevant areas of specific diseases, resulting in
lower validity. Disease-specific index measures on the other hand combine disease-specificity
and quantification of perceived quality on several health domains of a certain disease
into one single figure. These instruments have been developed for several diseases,
but a dementia-specific HRQoL index instrument was not yet available. Facing the increasing
individual and societal burden of dementia, specific HRQoL values with metric characteristics
are especially useful because they will provide vital information for health outcome
research and economic evaluations.

Aims of the study

To develop and validate the prototype of a dementia-specific HRQoL index measure:
Dementia Quality of life Instrument (DQI), as the first step towards valuation of
the dementia health state.

Methods

For development of the DQI we created a conceptual framework based on a review of
the literature, qualitative interviews with people with dementia and their carers,
expert opinion and team discussion. To assess validity we undertook a survey under
241 dementia professionals. Measurements consisted of ranking (1–5) and rating (1–10)
of 5 dementia-specific DQI domains (memory, orientation, independence, social activities
and mood) and simultaneously rating of 9 DQI-derived health states on a visual analogue
scale (VAS). We also performed a cross-sectional study in a large sample of people
with very mild to moderate dementia and their caregivers (N = 145) to assess feasibility
and concurrent validity. In addition, caregivers valued 10 DQI and 10 EQ-5D + C derived
health states of the patient simultaneously on the same VAS. Setting: outpatient clinics,
nursing homes and patient residences.

Results

All professionals judged the selected DQI domains to be relevant. Differences in ranking
and rating behaviors were small. Mood was ranked (≥3.3) and rated (≥8.2) as most,
orientation as least important (rank ≤2.6, value 7.5) health domain for dementia.
For the validation part of this study the completion rates for all domains were above
98% for patients and 100% for caregivers on patients. A priori hypothesized DQI versus
QOL-AD correlations that were significant in both patients and caregivers were: memory/memory,
orientation/memory, independence/physical health, social activities/energy and mood/mood.
Patient/caregiver inter-rater agreement was low (K < 0.2) for memory/independence,
fair (K 0.2-0.4) for orientation/mood, and moderate (K 0.4-0.6) for social activities.
Concurrent validity of the DQI with the EQ-5D + C was moderate. The fact that most
of the correlations between the domains of these two instruments were low (≤0.40)
showed that both instruments measure different elements of health status. As expected,
modest correlations (≥0.40) were observed between corresponding domains of the two
instruments.

Conclusions

Professionals judged all domains as relevant. The DQI prototype proved valid and feasible
for patients and caregivers and is appropriate for very mild to moderate dementia.
The differences in concurrent correlations with generic health status instruments
imply that the dementia-specific DQI health domains indeed provide different information.
The finding that patient HRQoL measured with the DQI was lower supports this notion.
The new DQI shows comparable psychometric properties to the best available dementia-specific
(QOL-AD) and generic (EQ-5D + C) measures. Further research is needed to generate
values in the general population for each of the possible DQI states and to derive
an algorithm that converts the 5 separate DQI domain scores into one single DQI Index
score. Introducing the DQI Index will advance dementia-related HRQoL measurement by
overcoming the shortcomings of generic and non-index instruments. This will allow
more unequivocal interpretation of subjective dementia HRQoL states in dementia research.